1. Field of the Invention
The present invention relates to an optical fiber amplifier, and more particularly, a long-band (1570 nm.about.1610 nm) optical fiber amplifier for providing enhanced power conversion efficiency by utilizing amplified spontaneous emission as a secondary pumping light source.
2. Description of the Related Art
In the past, erbium-doped fiber amplifiers were developed to amplify a single wavelength near the 1550-nm loss minimum of silica fibers. Then, with the advent of wavelength-division multiplexing, multiple wavelengths in that band can be amplified. In wavelength division multiplexed (WDM) optical communications system, one of important technological requirement is to provide an erbium-doped fiber amplifier (EDFA) with a flattened gain in a broad band since the multiwavelength operation adds some complications to the use of fiber amplifier.
In order to amplify the various wavelengths equally, some consideration has been given for obtaining an optical fiber amplifier to extend in the wide-band that the conventional EDFA cannot provide for. One of the methods for achieving the wide-band optical fiber amplifier is to use a new material for the optical fiber, such as the tellurite based optical fiber. Although tellurite based optical fiber amplifier has suitable properties to meet the requirements to be used as wide-band optical fiber amplifier, the relevant technologies have not been fully developed, and it has some undesirable characteristic, such as irregular gain spectrum.
Besides adopting such new materials, other efforts have been concentrated to achieve a gain in a band beyond the conventional amplified band (1530 nm-1560 nm; hereinafter referred to as "C-band"), by employing various designs of silica based EDFAs. In addition, there has been some proposed materials and structure to induce about 30 to 40% population inversion of the erbium atoms in the EDF to span the wavelengths from 1570 nm.about.1610 nm (hereinafter referred to as "L-band"). Using complex techniques, developers can run such C-band and L-band amplifiers in parallel so as to make the silica based EDFA having a wide gain band over 80 nm for the WDM transmission system. However, in order to develop such high capacity amplifier, the L-band fiber amplifier requires much longer length of EDFs and more powerful power pumps, which results in less power conversion efficiency in the fiber.
One significant concern with L-band fiber amplifiers is their power conversion efficiency. Also, of concern is the noise figure of the fiber amplifier. Thus, the present invention provides an optical power amplifier with improved power conversion efficiency, small-signal gain, and noise figure.